It is known in the art to selectively engage and disengage positive or jaw type clutches to shift a transmission from one speed ratio to another. Positive clutches are preferred in transmissions since they are compact and inexpensive relative to friction clutches and are extremely reliable if they are synchronously engaged. Most of these transmissions are manually shifted and employ a manually operated friction clutch for disconnecting the transmission input shaft from a prime mover when shifting from one ratio to another. Such disconnecting by the friction clutch has the disadvantage of interrupting the driving connection between the prime mover and the load connected to the transmission output shaft. Further, when such transmissions are used in relatively heavy vehicles, the vehicle operator must perform a double clutching manipulation of the friction clutch when shifting from one ratio to another. When double clutching, the operator must momentarily disengage the friction clutch to relieve torque on the positive clutch to be disengaged, then momentarily reengage the friction clutch to synchronize the positive clutch to be engaged, and then momentarily disengage the friction clutch prior to engagement of the positive clutch to minimize shock loading. Further, when the friction clutch is momentarily reengaged, the vehicle operator must either increase or decrease the prime mover speed to bring the positive clutch to synchronism. If the operator is unskilled or if the vehicle is moving slowly and/or if the vehicle is on a steep grade, it is not uncommon for a shift to be missed or for the positive clutches to be abused due to asynchronous engagement.
Many attempts have been made to adapt the above type of transmissions to automatic or semiautomatic controls to negate the above problems. One such attempt, as disclosed in U.S. Pat. No. 3,589,483, proposes positive clutches for engaging the several speed ratios, a first friction clutch for connecting the several speed ratio gears to a prime mover, a second friction clutch for connecting the transmission output shaft directly to the prime mover and synchronizing the positive clutches during upshifting, and a semiautomatic control system for controlling the sequential operation of the friction and positive clutches when a shift control lever is moved from one ratio position to another. When the lever is moved in an upshift sense, the control automatically provides power upshifting and synchronizing by momentarily or partially disengaging the first friction clutch and by momentarily or partially engaging the second friction clutch. When the lever is moved in a downshift sense, the control automatically provides a partial power downshifting by manipulating engagement and disengagement of the friction clutches, but synchronizing must be provided by engine speed manipulation. During both upshifting and downshifting, the clutch teeth of the positive clutch to be engaged are moved into abutment with each other prior to synchronism therebetween, thereby exposing the positive clutches to asynchronous engagement.